Use of fossil resources is inevitable in recent years. The fossil resources cannot be reproduced and may probably be exhausted in the near future. In an effort to solve the problem of fossil resource exhaustion, there have been developed techniques for controlling consumption of the fossil resources as chemical raw materials (resource saving techniques) and techniques for controlling consumption of the fossil resources as energy (energy saving techniques). Among the techniques for controlling consumption of the fossil resources as chemical raw materials, attention is paid to a technique of recycling plastics whose raw material is the fossil resources. However, plastics are more difficult to recycle than other materials such as metal and glass and it is the current situation that the plastics recycling technique has not yet been put into practical use. In particular, a thermosetting resin is neither softened or melted by heat nor dissolved in a solvent once it is cured. For this reason, it is technically difficult to regenerate, i.e., recycle, a cured article of a thermosetting resin into a raw material for plastics.
In order to overcome such a problem encountered in treating or recycling plastics, studies have been made for methods of solubilizing plastics by use of supercritical fluid or subcritical fluid. For instance, a method for solubilizing a thermosetting resin in a supercritical or subcritical solution of phenol compound or water/phenol compound has been investigated to solubilize and recycle a thermosetting resin which is difficult to solubilize with supercritical water (see, e.g., patent document 1: JP2001-151933A). With this method, it is possible to recover oligomer components by solubilizing a cured article of a thermosetting resin such as a phenol resin or the like. The oligomer components can be reused as a raw material for a thermosetting resin.
However, the technique set forth in the patent document 1 is directed to a production method that makes use of a batch type reactor. The batch type reactor suffers from problems of delay in heat transfer to the interior thereof and unevenness in reaction, especially in high-temperature and high-pressure reaction conditions. Thus, the batch type reactor is an arrangement ill-suited for a mass production. With regard to a recycling method, the patent document 1 refers to the reuse of the afore-mentioned oligomer components and mononuclear phenol but remains silent on a reuse method of a solid residue and the like contained in the reactants. In this viewpoint, it is hard to say that the patent document 1 discloses an arrangement suitable for mass production and commercialization.
As an improvement of the method for solubilizing a thermosetting resin in a supercritical or subcritical solution of phenol compound or water/phenol compound, there has been proposed a method of increasing a molecular weight of recoverable oligomer components by adding a formaldehyde compound and causing a polymerization reaction in the process of decomposing and solubilizing a thermosetting resin (see, e.g., patent document 2: JP2004-161983A). This method provides an advantage that it is possible to recover oligomer components of higher molecular weight merely by performing a solubilization treatment and further that the oligomer components can be reused in a broader range of applications.
However, the method specifically disclosed in the patent document 2 is also performed by a batch type reactor and therefore is not completely suitable for use in the mass production installation.
On the other hand, there has been disclosed a technique of continuously liquefying a cellulose-containing material at an elevated temperature (see, e.g., patent document 3: JP10-147786). This technique is directed to liquefaction of a naturally occurring material which can be relatively easily decomposed. Although the technique suggests a general methodology of the mass production, it fails to propose a concrete and practicable arrangement to an ordinary skilled person who intends to use the technique in decomposing and/or solubilizing plastics including a thermosetting resin.